The present invention relates to a polymer electrolyte fuel cell, particularly to a separator therefor.
A polymer electrolyte hydrogen-oxygen fuel cell is excellent in its output characteristics, whereby its application to e.g. automobiles, is expected. For practical application of the above fuel cell, it is required to develop a fuel cell which provides a high output density and a high energy efficiency constantly over a long period of time even under such an operational condition that the utilization ratios of a fuel and air are high.
The polymer electrolyte fuel cell usually has a construction such that a pair of power generation electrodes (a fuel electrode and an air electrode) face each other via a polymer electrolyte, and the pair of electrodes and the electrolyte are bonded to form a membrane electrode assembly. A plurality of such assemblies are laminated via separators, and the entire assembly is clamped to be integral (stacked).
Here, the separator has channels for supplying a fuel gas and an oxidant gas (such as air) to the electrodes, and it serves as a partition plate between the adjacent two units. Accordingly, the separator is required to have characteristics such that the gas permeability is low, it is light in weight, it is excellent in corrosion resistance and oxidation resistance when exposed to a steam atmosphere at a temperature of from room temperature to a vicinity of 150xc2x0 C. as the operational temperature of the fuel cell, it has good electrical conductivity for a long period of time, and it can be mechanically processable. Further, the separator is required to be a good conductor for electricity and heat in order to efficiently remove electricity and heat generated by a reaction of the cell out of the cell system.
As conventional separator materials, carbon type bulk materials such as artificial graphite and glassy carbon, are known. However, the carbon type materials are poor in toughness and brittle, whereby the following problems are likely to occur when it is used as a separator under such a condition that a stress other than a compression stress, or a mechanical shock, is likely to be exerted. Namely, the problems are such that the separator itself is broken, whereby the shape can not be maintained, cracking is likely to form, whereby air tightness can not be maintained, mechanical forming or processing is difficult, whereby the processing cost tends to be high, and recycling is difficult.
As a means to solve the above problems, it has been proposed to use as a separator a molded product obtained by subjecting flat graphite powder particles so called expanded graphite (such as Grafoil, tradename, manufactured by UCAR Co.) to dispersion treatment by e.g. an acid and adding a binder, followed by molding. This molded product is a flexible material which can be mechanically processable by e.g. pressing, whereby the problems relating to the toughness and the mechanical shock resistance, are overcome. However, such a molded product has a problem such that the mechanical strength is low, and the shape can hardly be maintained when made thin, or it is susceptible to deforming even when a small stress is applied thereto.
Further, the fuel cell is required to be small in size and light in weight and to provide a high output especially when it is used as a power source for an automobile, and it is necessary to increase a power per unit volume and to cool a heat generated by conducting electricity and by a reaction of the cell with a compact structure. Especially when a fluorine-containing polymer electrolyte having a high electrical conductivity, is used, this cooling will be essential, since the heat resistant temperature of the electrolyte is usually not so high. Cooling can be accomplished with the most compact structure when a fluid is permitted to flow at a high flow rate through a narrow and long channel formed in a separator, specifically, when water is permitted to flow under a high pressure. However, when the above-mentioned expanded graphite is used as the separator material, if the separator is maintained in the presence of water at a high temperature, the water is likely to penetrate into laminated particles of graphite, whereby the shape can not be maintained, thus leading to a problem that a long term reliability can not be attained.
As a means to solve the above-mentioned problems of the separator made of a carbon material, an attempt has been made to use a metal such as a surface-treated stainless steel, titanium or aluminum as the separator material (e.g. EP0780916). When a metal is employed, the mechanical processing will be easy, the strength will be high even thin, the toughness, the mechanical shock resistance, the fluid shielding property, and thermal and electrical conductivity will be excellent. However, the metal material has problems such that the specific gravity is large (for example, 8.0 with stainless steel, 4.5 with titanium and 2.7 with aluminum), whereby the output per unit mass of the fuel cell tends to be low.
Therefore, the present invention has an object to provide a polymer electrolyte fuel cell having a separator which is made of a material which can easily be formed and processed, whereby the shape and air tightness can be maintained even when a stress other than a compression stress, a mechanical shock, vibration, etc., are exerted, and the initial good electrical conductivity can be maintained over a long period of time even when exposed to a steam atmosphere from room temperature to the vicinity of 150xc2x0 C. as the operational temperature of the fuel cell, and which is light in weight and industrially practical.
The present invention provides a polymer electrolyte fuel cell comprising a plurality of membrane electrode assemblies laminated via separators, each assembly comprising a membrane-form polymer electrolyte and a pair of a fuel electrode and an air electrode facing each other via the electrolyte, wherein the separator has a fuel gas channel for supplying a fuel gas to the fuel electrode, an oxidant channel for supplying an oxidant to the air electrode and a fluid channel for removing a heat generated by a reaction out of the cell system, and the separator is made of a metal/non-metal composite material which has faces made of non-metal which are in contact with the membrane electrode assemblies and side walls of the fluid channel which are made of metal.
In the present invention, the separator has a role to provide a fuel gas and an oxidant to the fuel electrode and the air electrode, respectively, and a role to circulate a fluid for cooling to remove a heat generated by conduction of electricity and by a reaction of the cell out of the cell system. Here, air is used mainly as the oxidant, and accordingly, the electrode to which the oxidant is supplied, will be referred to as an air electrode in this specification. As the fluid for cooling, water is preferred, since the heat can thereby be efficiently removed. Further, the separator has a role to shield a gas or moisture to prevent permeation of the gas or moisture between the adjacent two membrane electrode assemblies and a role to transmit the generated electric current.
The fuel gas channel and the oxidant channel usually consist of grooves formed by ribs formed on the surface of the separator. Such a gas channel may consist of a plurality of linear or curved grooves, but a meandering groove may be formed over the entire surface of the separator so that it has only one inlet and one outlet for a gas.
In the present invention, the separator is made of a metal/non-metal composite material, and at least side walls of the fluid channel for cooling are made of metal. Accordingly, even when the above side walls are exposed to water as the fluid for cooling at a temperature of up to about 150xc2x0 C. as the operational temperature of the fuel cell, the water for cooling will not penetrate into the interior of the separator, and the shape of the separator can be maintained.
Further, in the separator in the present invention, at least faces which are in contact with the membrane electrode assemblies, are made of non-metal. The separator has a role to conduct an electric current, the above non-metal is preferably made of a highly electrically conductive carbon material. Particularly preferably, it is made of a molded product obtained by subjecting expanded graphite particles to dispersion treatment with e.g. an acid, adding a binder, followed by molding. Such a molded product is readily mechanically processable by e.g. pressing and is light in weight.
In order to reduce the weight of the separator in the present invention, it is preferred that the proportion of the non-metal in the metal/non-metal composite material is as large as possible. Accordingly, from this viewpoint, it is preferred that all other than the side walls of the fluid channel is composed of the non-metal. However, from the production efficiency of the separator and high strength of the separator, it is preferred that a layer made of metal having a fluid channel internally (hereinafter referred to as a metal layer) is sandwiched between a pair of layers made of non-metal having gas channels on their surfaces (hereinafter referred to as non-metal layers) and bonded to constitute the separator. Here, the pair of non-metal layers sandwich the metal layer so that the gas channels will be located on the surfaces.
In this case, the non-metal layers may be the above-mentioned molded products composed of expanded graphite particles. However, they may be those formed on the metal layer by a printing method or a coating method by using a conductive paste containing a highly electrically conductive carbon material.
In the present invention, the metal constituting the separator is preferably one member selected from the group consisting of a metal containing aluminum in an amount of at least 80% of the total mass of metal, a metal containing titanium in an amount of at least 80% of the total mass of metal and stainless steel. Here, the metal includes an alloy. These metals have high strength even when they are thin, and they are excellent in reliability against a dynamic shock such as vibration or mechanical shock and against a static mechanical load such as tension or compression. Further, they are capable of maintaining their shapes even when exposed to a fluid such as water at a high temperature for a long period of time, and they are excellent also in thermal and electrical conductivity.
In the above-mentioned metal containing aluminum or the metal containing titanium, if the content of aluminum or titanium as the main component, is less than 80%, the specific gravity of metal constituting the separator tends to be large, such being undesirable. The content of aluminum or titanium is particularly preferably from 90 to 98%.
The metal containing aluminum as the main component is light in weight, easy for recycling and mechanically readily processable, such being desirable. As the aluminum alloy, an alloy of aluminum with at least one metal selected from the group consisting of magnesium, manganese, silicon, copper, nickel, lithium, zinc, lead, bismuth, titanium and tin, is preferred. For example, duralumin, silumin, hydrosodium or anticorroidal may be mentioned.
Further, the metal containing titanium as the main component has high mechanical strength per unit weight and relatively high corrosion resistance, such being desirable. For example, an alloy of titanium with at least one metal selected from the group consisting of aluminum, iron, vanadium, molybdenum, manganese, chromium, zirconium, tin, silicon, palladium and tantalum, is a corrosion resistant alloy and preferred.
Further, stainless steel has high mechanical strength per unit volume and relatively high corrosion resistance, such being desirable. The stainless steel is not particularly limited, and any one of austenite type, ferrite type and martensite type, may be employed. From the viewpoint of corrosion resistance, austenite type stainless steel is particularly preferred.
It is preferred that a coating film containing ceramics and having a resistivity of at most 3xc3x9710xe2x88x924 xcexa9xc2x7cm, particularly from 3xc3x9710xe2x88x926 to 1xc3x9710xe2x88x924 xcexa9xc2x7cm, is formed on the surface of side walls of the fluid channel made of the metal, i.e. on the surface of the above metal which is in contact with the fluid. By the formation of the coating film containing ceramics, deterioration of the metal surface by oxidation can be prevented. If the resistivity of the coating film exceeds 3xc3x9710xe2x88x924 xcexa9xc2x7cm, the resistivity of the overall fuel cell tends to be high, whereby it may happen that the energy can not efficiently be taken out. This coating film may be a coating film composed solely of ceramics, or a coating film having ceramics dispersed in metal.
Further, it is preferred that a layer containing ceramics and having a resistivity of at most 3xc3x9710xe2x88x924 xcexa9xc2x7cm, is disposed at the interface between the metal and the non-metal constituting the separator. In the polymer electrolyte fuel cell, water is formed at the air electrode along with the reaction of the cell. Further, usually, in order to maintain the ion conductivity of the polymer electrolyte, the fuel gas and the oxidant gas are supplied to the respective channels, as wetted so as not to dry the electrolyte. Such formed water and the steam contained in the gas will not flow rapidly and in a large amount like water as the fluid for cooling, whereby even if the non metal portion of the separator is exposed to them, the shape of the separator will not be disintegrated.
However, such water contents are likely to penetrate through the non metal portion of the separator and reach the interface between the metal and the non-metal. Consequently, it is likely that the surface of the metal in contact with the non-metal will be oxidized, whereby the electrical conductivity tends to decrease. Here, if a layer made of ceramics excellent in the durability as compared with the metal, is present at the interface between the metal and the non-metal, deterioration of the metal can be prevented. Further, if the resistivity of this layer made of ceramics exceeds 3xc3x9710xe2x88x924 xcexa9xc2x7cm, the resistivity of the overall fuel cell tends to be high, whereby the energy may not efficiently be taken out. The resistivity of this layer is particularly preferably from 3xc3x9710xe2x88x926 to 1xc3x9710xe2x88x924 xcexa9xc2x7cm. This layer may be a layer composed solely of ceramics or a layer having ceramics dispersed in metal.